The invention relates generally to semiconductor device fabrication and, in particular, to heterojunction bipolar transistors and fabrication methods for a heterojunction bipolar transistor.
Bipolar junction transistors may be found, among other end uses, in high-frequency and high-power applications. In particular, bipolar junction transistors may find specific end uses in amplifiers for wireless communications systems and mobile devices, switches, and oscillators. Bipolar junction transistors may also be used in high-speed logic circuits. Bipolar junction transistors are three-terminal electronic devices that include an emitter, an intrinsic base, and a collector defined by regions of different semiconductor materials. In the device structure, the intrinsic base is situated between the emitter and collector. An NPN bipolar junction transistor may include n-type semiconductor material regions constituting the emitter and collector, and a region of p-type semiconductor material constituting the intrinsic base. A PNP bipolar junction transistor includes p-type semiconductor material regions constituting the emitter and collector, and a region of n-type semiconductor material constituting the intrinsic base. In operation, the base-emitter junction is forward biased, the base-collector junction is reverse biased, and the collector-emitter current may be controlled by the base-emitter voltage.
Device structures and fabrication methods are needed that improve the performance of a bipolar junction transistor.